The present invention relates to a planet gear type torque transmission device for use in a starter for vehicles or the like.
Previously known planet gear type torque transmission devices are disclosed in e.g. Postexamined Japanese Utility Model Publication (Kokoku) Hei-2-7262 and Unexamined Japanese Utility Model Publication (Kokai) Hei-4-121469.
First, respective components in FIGS. 9 to 13 for explaining the prior will be explained. Reference numeral 1 denotes an internal deceleration type starter including a planet gear type deceleration device 2 (described later) serving as a torque transmission device. Numeral 3 denotes a front machine frame externally fixed. Numeral 4 denotes an internal gear mounted on the inside 3a of the front machine frame 3. This internal gear is integrally formed by high-polymer synthetic resin) by molding. Numerals 5 denote two concave portions which are provided symmetrically in the outer periphery 6 (described later) of the internal gear 4. The internal gear 4 is set in the front machine frame when the concave portions 5 are fit in the convex portions 3b formed in the front machine frame 3. Numeral 6 denotes the outer periphery of the thin internal gear. Numeral 7 denotes a rib serving as a pillar coupling portion for coupling the outer periphery 6 and an inner periphery 13 (described later). Numeral 9 denotes a through-hole formed by the outer periphery 6, rib 7 and inter periphery 13. A through bolt (not shown) penetrates through the through-hole.
A flange portion 10 is provided at the right end of the internal gear 4 in FIG. 9. The inside of the flange portion 10 constitutes a boss portion 11. A sleeve bearing 12 is fit in the inside of the boss portion 11. Internal teeth 14 are successively provided at regular intervals in the inner periphery 13 of the internal gear 4. A planet gear 15, which abuts on the insides of the internal teeth 14, engages the internal teeth 14. The planet gear 15 has a through-hole 16. The planet gear 15 is supported by a supporting pin 17 fit in the through-hole 16 through the sleeve bearing 18. Since the supporting pin 17 is fit in the output rotary shaft flange portion 20 integrally formed on the left end of an output rotary shaft 19 in FIG. 9, the revolving force of the planet gear 15 is transmitted to the output rotary shaft 19. The output rotary shaft 19 is rotatably supported by the flange portion 10 through the sleeve bearing 12.
The planet gear 15 also engages a spur gear 22 attached to the input rotary shaft 21 on the side opposite to the side where it engages the internal teeth 14. The input rotary shaft 21, which is the rotary shaft of a DC motor (not shown), provides a revolving force to the planet gear 15 through the spur gear 22. The input rotary shaft 21 and output rotary shaft 19 are made rotatable by the sleeve bearing 23 and a rotary ball 24. Incidentally, numeral 25 denotes an iron for jointing the DC motor (not shown) to the front machine frame 3. In the arrangement described above, the planet gear deceleration device 2 is composed of the internal gear 2, planet gear 4 and spur gear 22.
In operation, when the input rotary shaft 21 rotates by energization of the DC motor (not shown), this rotation is supplied to the planet gear deceleration device 2. Thus, the rotation with reduced rotary speed and enhanced rotary force will be transmitted from the planet gear deceleration device 2 to the output rotary shaft 19.
In fabrication of the internal gear 4, a mold is used for molding high-polymer synthetic resin. When the mold has a shape not collected as indicated by a solid line in FIG. 12, in the fabrication process, thermal shrinkage of the high-polymer synthetic resin called "mold reduction" occurs. Particularly, the mold reduction occurs in the directions of arrows a at the ribs 7 in FIG. 12. As a result, the ribs 7 are distorted as indicated in one-dot chain line in FIG. 12 after molding. In order to obviate such an inconvenience, correction indicated by a solid line in FIG. 13 is previously added to the mold. After molding, the mold reduction leads to the internal gear 4 having the shape indicated by a solid line in FIG. 13.
In the planet gear type torque transmission device as described above, in integrally forming the internal gear by resin molding, the mold reduction due to the shrinkage of the ribs occurs so that the internal teeth are deformed. This required the mold to be modified in forming the internal gear by resin molding.
Further, since the ribs provided in the outer periphery of the internal gear are irregularly located with respect to the internal teeth, deformation of the internal teeth due to the mold reduction for the ribs occurs irregularly. This makes it impossible to perform regular correction for the internal teeth portion of the mold, thus making it difficult to increase the accuracy of the internal teeth.